I received my Lightning back from BSA today. It would appear that all they have done is chrono it, and fire a test group which apparently was 1/2" at 25 yards. Neither of my concerns regarding the tightness at the breech or the natural of the internal lubrication have been addressed. After speaking to BSA, I don't believe the rifle was disassembled and checked internally in any way.
While I'm disappointed, I'm not surprised. I've subsequently contacted BSA via email, the body of which is below. I hope for a positive response and I'll let you know how I get on!

Dear Sir,

I recently purchased a BSA Lightning XL SE GRT from Protek Supplies in Bognor Regis who have been most helpful in helping me resolve some issues I have with the rifle.
During routine maintenance (simply removing the action from the stock to clean and protect the underside of the action after use outside), I noticed two points that I wish to raise with you. The rifle has already been returned to yourselves with a covering note outlining my concerns. The rifle was tested, found to fall within your criteria and returned with no remedial action taken.

My first concern is with the barrel. My particular example exhibits a tightness at the breech for approximately 20-25mm, after which the pellet can be pushed down the bore with minimal pressure before reaching the choke. This is far more noticeable with pellets that have a larger head, for example Wembley Accupell or Powerpell at 5.51mm, than with something like AA Diabolo Field at 5.50mm. Various sources, including staff at Sandwell Field Sports have confirmed that this apparent restriction at the breech should not be the case, and can have a detrimental effect on accuracy and the nature of the shot cycle.
The note on your repair form (#6206) stated, "Our barrels have a tighter muzzle as they are hammer swaged." but made no mention of the tightness at the breech.
Having examined a friends Lightning XL Tactical in the same calibre, it doesn't exhibit the same tightness at the breech and is consistently more accurate with a range of pellets in a back to back test with my example, although being a conventional spring rather than a gas strut may have a bearing in this. It would however be disappointing if the more expensive GRT was inherently less accurate for whatever reason.

The second concern is with the nature of the internal lubrication as supplied. I completely understand that the gas spring is capable of long and trouble free service with the absolute minimum of a light lubricating oil. But the same can not be said for other bearing surfaces within the cylinder, for example the rear of the piston skirt to the inside of the cylinder during the high, off axis force associated with the cocking stroke. There is no evidence of any lubrication in this area other than a thin application of light oil. A swab of this area taken through the cocking slot revealed small particles of metal which would suggest that wear is taking place, even after only a few hundred cycles. As someone with an engineering background, I would be concerned that a steel to steel bearing face lubricated with just a static film of light oil would be subject to levels of wear that could render the components not fit for purpose within the warranty period. This concern was outlined in the covering note and was addressed with a brief note stating, "A dry system of operation. The only part that is oiled is the piston seal and cylinder."
The use of just oil in this area would seem to be at odds with accepted good practise regarding the internal lubrication of spring/ gas ram air guns. While I am quite happy and equipped to remedy this myself, to do so would involve the dismantling of the rifle thus invalidating the warranty. To not do so will cause considerable problems with worn components that will considerably curtail my enjoyment and the longevity of my purchase.

I bought this rifle because it's combination of performance, appearance, weight and dimensions made it really stand out in is what quite a crowded market. At the moment I am somewhat disappointed, and I sincerely hope that these issues can be resolved so that I can recommend your product to anyone and everyone else.

Best regards,